Menu

Let’s say that you have a running Flask server and you want your user to control the state (on/off) of an LED via the GPIO. The following technique shows how to connect Flask server with a separate GPIO script using PID.

Let’s say that you have a running Flask server and you want your user to control the state (on/off) of a motion sensor via the GPIO. The most complex way is to create a multi-threading script which handles the server and GPIO code.

Another approach is to separate the server script from the GPIO script. Thus having two layers of scripts. This has the advantage to debug in isolation. For this technique one has to use Unix Signals which can be used to send signals from one process to another.

When you execute a script on your UNIX system, the system creates a process id (pid) which is different every time. A signal is a software interrupt which notifies a process with a significant event or request.

The following table gives a list of the most common signals:

NAME

NUMBER

DESCRIPTION

SIGHUP

1

Linux sends a process this signal when it becomes disconnected from a terminal.

SIGINT

2

Linux sends a process this signal when the user tries to end it by

pressing CTRL+C.

SIGILL

4

Linux sends a process this signal when it attempts to execute an illegal instruction.

SIGABRT

6

Linux sends a process this signal to the process when the process calls the ‘abort ()’ function

SIGFPE

8

Linux sends a process this signal when it has executed an invalid floating-point math instruction

SIGKILL

9

Linux sends a process this signal to end it immediately

SIGUSR1

10

User programs can send this signal to other process

SIGUSR2

12

User programs can send this signal to other process

SIGSEGV

11

Linux sends a process this signal when the program has attempted an invalid memory access

SIGPIPE

13

Linux sends a process this signal when the program has attempted to access a broken data stream, such as a socket connection that has been already closed

SIGALRM

14

A process can receive this signal from the Linux using the function alarm (), after a time period mentioned in its argument.

SIGTERM

15

Linux sends a process this signal requesting it to terminate

SIGCHLD

17

Linux sends a process this signal when a child process exits

SIGXCPU

24

Linux sends a process this signal when it exceeds the limit of

CPU time that it can consume.

SIGVTALRM

26

A process can receive this signal from the Linux using the function setitimer (), after a time period mentioned in its argument.

We are interested in SIGUSR1 and SIGUSR2 which can be used to send user signals.

Step 1:

First create the server layer (app.py). Documentation about Flask server can be found here.

Download

This is a brief explanation how to create a Christmas tree that its LEDs are controlled by the end user from a website. The concept is pretty simple. We need to have a server, a website, a database, a relay, LEDs and of course a Netduino. Obviously this concept can be applied on a RaspberryPi.

Video Demonstration

General Setup

For this project Node.JS was chosen because it has a popular library called Socket.IO and can be easily connected with MongoDB. Socket.IO is a JavaScript library for real time web applications. It enables real-time communication between web clients and servers. MongoDB is a cross-platform document-oriented database which is classified as a NoSQL database. Socket.IO was used to display the Christmas tree’s animation state (controlled by netduino) in real time to all connected users via Node.JS. MongoDB was used to store the user’s LEDs animation.

Step 2 – Upon submission, the user’s data is stored in MongoDB and emitted via Socket.IO to all connected clients in JSON.

Step 3 – Netduino checks every two seconds with Node.JS if there’s a new animation for playing. If there is, Node.JS gets the data from the database and returns it to Netduino as JSON string. Then Netduino parses the JSON string and plays the animation. Once it finishes, it instructs Node.js to get the next animation.